1. Field of the Invention
The present invention relates to a stock liquor pressure pulsation absorbing apparatus and method for carrying out an active absorption of low frequency pressure pulsation in a stock liquor piping system.
2. Description of the Prior Art
Prior art of the above-mentioned type will be described with reference to FIGS. 8 and 9.
FIG. 8 shows schematically a stock liquor flow passing through an attenuator, which is widely used in the prior art. Stock liquor is fed into a master pipe 03 by a fan pump 01 via a screen 02 to flow through an attenuator 04, which is disposed midway along the master pipe 03. The stock liquor enters a headbox 08, is then injected onto a wire 09 to enter a paper forming process, and is formed into paper via a downstream dewatering process and drying process which are not shown.
The attenuator 04 consists of a portion of the master pipe 03, an air chamber 05 which is partitioned from the master pipe 03 by a diaphragm 06, etc. The air chamber 05 communicates with a volume tank through a passage 07, so that pressure in the air chamber 05 is set to be equal to a mean pressure in the master pipe 03 by a controller which is not shown.
Thus, when a pulsation occurs which is higher than the mean pressure in the master pipe 03, the pressure in the master pipe 03 becomes higher than the pressure in the air chamber 05, and the diaphragm 06 is pressed down. As a result, the stock liquor, in an amount corresponding to the volume increase in the master pipe 03 due to displacement of the diaphragm 06, is absorbed, and flow variation in the master pipe 03 of the stock liquor flowing into the headbox 08 is mitigated.
When a pulsation occurs which is lower than the mean pressure in the master pipe, the diaphragm 06 is pushed up and the pulsation is absorbed. That is, the attenuator in the prior art is a pressure pulsation absorbing apparatus of a passive type in which the pulsation is absorbed by a differential pressure between the pressure in the master pipe 03 and the internal pressure of the air chamber 05.
Also, Japanese laid-open patent application No. Hei 1(1989)-298291 discloses an apparatus in which water is supplied into stock liquor piping. The flow rate of the water is controlled, whereby pulsation in the stock liquor piping is absorbed.
An outline of this apparatus is shown in FIG. 9, wherein a pressure gauge 016 is disposed downstream of a joining point of stock liquor piping 015 and a water supply pipe 014. A valve 011 is opened and closed by a pressure signal taken from the pressure gauge 016, and thus the flow rate of the water flowing into the stock liquor piping 015 from the water supply pipe 014 is controlled. It is mentioned there that a pulsation of 1 to 50 Hz can be absorbed by this apparatus.
In this apparatus, however, if a low frequency pulsation of 1 Hz or less is to be absorbed, it is necessary to supply a large amount of water, and a possibility occurs of causing a consistency variation of the stock liquor in the piping. It can be said, therefore, that absorption of the low frequency pulsation will be very difficult.
It is to be noted in FIG. 9 that numeral 012 designates a controlling member for operating the valve 011, numeral 013 designates a servo valve, numeral 017 designates an electronic regulator for transmitting a command of the pressure gauge 016, numeral 018 designates a water tank, numeral 019 designates an operating oil tank and numeral 020 designates a headbox.
Generally, in order to obtain a very stable operation of a paper machine, it is necessary to make a pressure pulsation level within .+-.0.5 to .+-.1% or less of a mean pressure in a range of 0.01 to 100 Hz. In an ordinary paper making plant, however, such a severe permissible pressure pulsation level will hardly be secured even if the design of the piping systems therefor is done with full deliberation.
As the frequency of pressure pulsation becomes lower, flow variation becomes larger, generally, and the stock liquor absorption amount at the attenuator increases. Because the attenuator is of a structure to absorb variation in the flow rate of the stock liquor using a diaphragm, in order to absorb a pulsation of low frequency, especially of 1 Hz or less, it is necessary to enlarge an area and displacement amount of the diaphragm. However, there are restrictions from the structure, installation space, etc., and hence a pulsation absorption performance at 1 Hz or less has been inevitably lowered. Moreover, pulsation absorption of 1 Hz or less has been very difficult not only simply due to this facilities-related reason, but from a functional point of view also.
The above problems are applicable to the apparatuses shown in FIGS. 8 and 9 generally, and as to the apparatus shown in FIG. 9 also, there are considered additional problems in securing water to be supplied for the controlling, handling and treating of the water, controlling the stock liquor consistency after water is added, etc., and an actual apparatus will be hardly realized.